A doppler effect and a multi-path phenomenon of a wireless channel cause amplitude phase time varying distortion and inter-code interference of a received signal. A multi-carrier communication system (OFDM and SC-FDM) uses a cyclic prefix to obtain a valid anti inter-code interference capability; and a multi-antenna (MIMO) system uses the independence of the decline between antennae to increase link stability or improve system capacity. The combination of the two greatly develops features of the wireless channel, and turns a defect thereof into a favourable factor which can improve capacity; therefore, a multi-carrier MIMO system is widely adopted in a new generation of mobile communications (such as LTE and WiMax).
For recovering a transmitting signal successfully, channel estimation is a step which is necessary in a multi-carrier MIMO system receiver. The channel estimation of the multi-carrier system is mainly divided into two categories, blind channel estimation and pilot assisted channel estimation, wherein the pilot assisted channel estimation may obtain higher estimation accuracy and stability, and is thus widely adopted in a practical system. Different from the pilot assisted channel estimation in a traditional multi-carrier system, the multi-carrier MIMO system relates to acquisition of channel gains of paths among multiple antennae; therefore, it is necessary to design a special pilot arrangement solution to avoid interference among multi-antennae; and the disclosure adopts the following frequency domain code division pilot frequency design:
When the number of transmitting antennae is NT, assuming that the number of sub-carriers occupied by a user is K, a time domain pilot sequence xi(n), i=1, 2, . . . , NT, n=1, 2, . . . , K is firstly generated, wherein xi(n) is (i−1)K/NT cyclic shift version of x1(n), i.e., xi(n)=x1(n−(i−1)K/NT|mod K); and NT time domain sequence are respectively transformed into frequency domain sequence, and are recorded as a diagonal form to obtain xi, i=1, 2 . . . , NT, and Xi=XiΛi, wherein Λi is a diagonal matrix, the lth diagonal element is:
            Λ      i        ⁡          [              l        ,        l            ]        =            exp      ⁡              (                  j2π          ⁢                                    (                              i                -                1                            )                                      N              T                                ⁢                      (                          l              -              1                        )                          )              .  
In addition, the pilot time domain and frequency domain sequences are both constant modulus sequences, such as ZC. After zero-padding, a frequency domain pilot signal is extended to N, and is sent, after an IFFT and an adding CP operation, to a RF antenna to perform operations such as D/A and up-conversion, so as to be transmitted.
Any antenna at a receiving end obtains a digital receiving signal via down-conversion and A/D sampling; and after a CP being removed, an N-point FFT, and an demultiplexing operation, the frequency domain received pilot signal may be obtained and is:
      Y    =                                        ∑                          i              =              1                                      N              T                                ⁢                                    X              i                        ⁢                          H              i                                      +        N            =                                    ∑                          i              =              1                                      N              T                                ⁢                                    X              1                        ⁢                          Λ              i                        ⁢                          H              i                                      +        N              ,
where Hi is a channel gain from a transmitting antenna i to a receiving antenna, having a frequency domain autocorrelation matrix RHi, and N is white Gaussian noise, having a frequency domain autocorrelation matrix N0I, wherein N0 is white noise power, and I is an identity matrix.
Firstly, three pilot frequency-based channel estimation technologies in a common OFDM (also applicable to SC-FDM) currently will be introduced:
Least square (LS) channel estimation: LS channel estimation is the basest and simplest channel estimation method in the OFDM system. In the method, the direct inverse multiplication of a receiving signal at a pilot frequency sub-carrier location and a pilot frequency signal is taken as an estimation value of a channel; and LS channel estimation is also a basic step of other more complex channel estimation technologies. LS channel estimation has the lowest calculation complexity; however, the method does not consider deterioration of the estimation performance caused by the noise and interference between antennae at all; and once the method is adopted, it may significantly influence system performance.
Discrete Fourier transform-based (DFT-Based) channel estimation: DFT-Based channel estimation performs noise filtering and interference suppression by using the feature that vector energy of a time domain channel is concentrated. Frequency domain LS channel estimation is firstly performed; then a channel frequency domain response value is transformed into a time domain response value via IDFT, to perform windowing on an impulse response of the channel so as to suppress noise; and finally, the channel frequency domain response value is transformed into a frequency domain response value, so as to obtain a final channel frequency domain response estimation value. Since the DFT operation has a rapid computer execution method and the DFT-Based technology has lower complexity, the windowing operation may suppress noise and interference between antennae effectively. However, leakage phenomenon of time domain channel energy causes the technology to inevitablely suffer the influence of losing useful information and remaining interference between antennae.
Linear minimum mean squared error (LMMSE) channel estimation: an LMMSE algorithm uses channel statistic information to obtain linear estimation of a minimum mean squared error, and is a linear optimal estimation algorithm. The LMMSE algorithm has a higher calculation complexity, and needs the channel statistic information to perform estimation; and this is hard to be obtained in practice.
The embodiments of the disclosure provide a practical LMMSE channel estimation technology, i.e., PLMMSE (Practical LMMSE), which is a channel estimation algorithm that can be achieved in practice, and can obtain accurate channel estimation in a case where multi-antenna of a transmitter adopts frequency domain code division multiplexing.